1. Field of Invention
The invention relates to a driving circuit for a liquid crystal display (LCD), and more particularly to a driving circuit having a plurality of switches disposed between a negative input terminal and an output terminal of an OP amplifier.
2. Related Art
A LCD typically needs a source driving circuit for driving a source of a thin film transistor (TFT) so as to control a transmission rate of the TFT. FIG. 1 shows a circuit of a driving unit 10 in a conventional driving circuit. Referring to FIG. 1, the driving unit 10 includes two OP amplifiers 11 and 12 and two switches 13 and 14. The positive input terminal of the first OP amplifier 11 receives a first analog signal outputted from a positive digital-to-analog converter (PDAC), and the output terminal of the first OP amplifier 11 is connected to an output node OUT through the first switch 13. The positive input terminal of the second OP amplifier 12 receives a second analog signal outputted from a negative digital-to-analog converter (NDAC), and the output terminal of the second OP amplifier 12 is also connected to the output node OUT through the second switch 14. In addition, the output terminal of each of the OP amplifiers 11 and 12 is directly fed back to the negative input terminal of the OP amplifier.
FIG. 2 shows another circuit of a driving unit 20 of the conventional driving circuit. As shown in FIG. 2, the driving unit 20 includes two OP amplifiers 11 and 12 and two switches 13 and 14, and further includes a third switch 13′ and a fourth switch 14′. The connections between the OP amplifiers 11 and 12 and the two switches 13 and 14 of the driving unit 20 are the same as those of the driving unit 10 of FIG. 1. That is, the OP amplifiers 11 and 12 are connected to a first output node OUT_ODD through the switches 13 and 14. But the driving unit 20 further connects the output terminal of the first OP amplifier 11 to a second output node OUT_EVEN through the third switch 13′, and connects the output terminal of the second OP amplifier 12 to the second output node OUT_EVEN through the fourth switch 14′.
However, the driving unit in either FIG. 1 or FIG. 2 has the equivalent circuit shown in FIG. 3 after the switch is turned on. So, the equivalent output impedance Rout viewed from the output node into the OP amplifier is:Rout=Ron(OP)+Ron(Switch)  (1).
Because the output impedance Rout is increased due to the switch, the time of the stable output voltage of the output node is thus influenced so that the response speed of the LCD cannot be effectively enhanced.